Everything Science Grade 11

9.7 CHAPTER 9. THE LITHOSPHERE

9.7 Mining and mineral processing: Phosphates

ESBCV

A phosphate is a salt of phosphoric acid (H 3 PO 4 ). Phosphates are the naturally occurring form of the

element phosphorus. Phosphorus is seldom found in its pure elemental form, and phosphate therefore

refers to a rock or ore that contains phosphateions. The chemical formula for the phosphate ion is

PO^34 −.

Mining phosphates ESBCW

Phosphate is found in beds in sedimentary rock,and has to be quarried toaccess the ore. A quarryis a

type of open pit mine that is used to extract ore.In South Africa, the mainphosphate producer is at the

Phalaborwa alkaline igneous complex, which produces about 3 milliontons of ore per year. Theore is

crushed into a powder and is then treated with sulphuric acid to form a superphosphate (Ca(H 2 PO 4 ) 2 ),

which is then used as afertiliser. In the equationbelow, the phosphate mineral is calcium phosphate

(Ca 3 (PO 4 ) 2.

Ca 10 (PO 4 ) 6 F 2 + 7H 2 SO 4 + 3H 2 O→ 3Ca(H 2 PO 4 ) 2 H 2 O + 7CaSO 4

Alternatively, the ore can be treated with concentrated phosphoric acid(which forms a triple super-

phosphate), in which case the reaction looks like this:

Ca 10 (PO 4 ) 6 F 2 + 14H 3 PO 4 + 10H 2 O→ 10Ca(H 2 PO 4 ) 2 H 2 O + 2HF

Uses of phosphates ESBCX

Phosphates are mostly used in agriculture. Phosphates are one ofthe three main nutrientsneeded by

plants, and they are therefore an important component of fertilisers to stimulate plant growth.

Exercise 9 - 4

Rock phosphate [Ca 10 (PO 4 ) 6 F 2 ], mined from open pit mines at Phalaborwa, is animportant raw mate-

rial in the production offertilisers. The followingtwo reactions are used to transform rock phosphate

into water soluble phosphates:

A: Ca 10 (PO 4 ) 6 F 2 + 7X + 3H 2 O→ 3Ca(H 2 PO 4 ) 2 H 2 O + 2HF + 7CaSO 4

B: Ca 10 (PO 4 ) 6 F 2 + 14Y + 10H 2 O→ 10Ca(H 2 PO 4 ) 2 H 2 O + 2HF

1. Identify the acids represented by X and Y.


2. Despite similar molecular formulae, the products Ca(H 2 PO 4 ) 2 formed in the two reactions have
   different common names. Write down the names for each of these products in reactions A and
   B.